Water based polyester insulating coating compositions, using aromatic and aliphatic dicarboxylic acids, alicyclic dihydric alcohol, and tris (2 hydroxy alkyl) isocyanurate, with methylated melamine formaldehyde and aliphatic amine, are taught by Thomas et al., in U.S. Pat. No. 4,130,520. While these water based insulating compositions eliminated fire and health problems that could be associated with organic solvent based polyesters, there is a need for completely solventless, insulating, thin film coating compositions, that could be used as dipping and vacuum impregnating resins for insulation used in various electrical apparatus.
Fekete et al., in U.S. Pat. No. 3,533,999, teaches solventless polyester casting compositions having good electrical and thermal properties, as replacement for wood, ceramics and rubber in electrical applications. These compositions contain unsaturated dicarboxylic acid and maleic anhydride added to a mixture of ethylene or propylene glycol with neopentyl glycol, and hydroquinone as sole inhibitor, all of which may be diluted with styrene or vinyl toluene. Schroeter et al., in U.S. Pat. No. 3,875,094, teaches UV curable, wax containing, solventless polyester coating compositions having good electrical and hydrolysis properties. These compositions contain a mixture of ethylene or propylene glycol with neopentyl glycol mixed with fumaric acid, and quinone or catechol as sole inhibitor, all of which may be diluted with styrene or vinyl toluene.
While these systems solved various problems in the insulation industry, what is still needed is a polyester system specialized for dipping of random wound coils for electrical apparatus, or vacuum impregnation of form wound coils for electrical apparatus. It is essential that a dipping varnish have long storage stability, since it may be used in 500 to 4,000 gal. quantities in dipping tanks. This specialized varnish should also have a low viscosity, short gel time, good moisture resistance, high flash point, low power factor at elevated temperatures, and provide a good build after a single dip.